Short cavity tunable laser with mode position compensation

ABSTRACT

A semiconductor tunable laser system includes a tunable Fabry-Perot cavity and a cavity length modulator, which controls an optical length of the cavity at least over a distance corresponding to the spacings between the longitudinal modes of the laser cavity. Thus, the tunable Fabry-Perot cavity allows the laser cavity to have gain at the desired wavelength of operation while the cavity length modulator tunes the cavity length such that a longitudinal cavity mode exists at the desired wavelength of operation. Also, in one embodiment, a wavelength locker system is further provides that has a differential wavelength filter, e.g., stepped etalon, and a multi-element detector, e.g., a quad-detector. The controller then modulators the Fabry-Perot cavity to control the wavelength in response to the signal received from the multi-element detector.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing (WDM) systems typically comprisemultiple separately modulated laser systems at the transmitter. Theselaser systems are designed or actively tuned to operate at differentwavelengths. When their emissions are combined in an optical fiber, theresulting WDM optical signal has a corresponding number of spectrallyseparated channels. Along the transmission link, the channels aretypically collectively amplified in semiconductor amplifier systems orgain fiber, such as erbium-doped fiber and/or regular fiber, in a Ramanamplification scheme, although semiconductor optical amplifiers are alsoused in some situations. At the receiving end, the channels are usuallyseparated from each other using, for example, thin film filter systemsto thereby enable detection by separate detectors, such as photodiodes.

The advantage of WDM systems is that the transmission capacity of asingle fiber can be increased. Historically, only a single channel wastransmitted in each optical fiber. In contrast, modern WDM systemscontemplate hundreds of spectrally separated channels per fiber. Thisyields concomitant increases in the data rate capabilities of eachfiber. Moreover, the cost per bit of data in WDM systems is typicallyless than comparative non-multiplexed systems. This is because opticalamplification systems required along the link is shared by all of theseparate wavelength channels transmitted in the fiber. Withnon-multiplexed systems, each channel/fiber would require its ownamplification system.

Nonetheless, there are challenges associated with implementing WDMsystems. First, the transmitters and receivers are substantially morecomplex since, in addition to the laser diodes and receivers, opticalcomponents are required to combine the channels into, and separate thechannels from, the WDM optical signal. Moreover, there is the danger ofchannel drift where the channels lose their spectral separation andoverlap each other. This interferes with channel separation anddemodulation at the receiving end.

Minimally, the optical signal generators, e.g., the semiconductor lasersystems that generate each of the optical signals corresponding to theoptical channels for a fiber link, must have some provision forwavelength control. Especially in systems with center-to-centerwavelength channel spacings of less than 1 nanometer (nm), the opticalsignal generator must have a precisely controlled carrier wavelength.Any wander impairs the demodulation of the wandering signal at the farend receiver since the wavelength is now at a wavelength different thanexpected by the corresponding optical signal detector, and the wanderingsignal can impair the demodulation of spectrally adjacent channels whentheir spectrums overlap each other.

In addition to wavelength stability, optical signal generators that aretunable are also desirable for a number of reasons. First, from thestandpoint of manufacturing, a single system can function as thegenerator for any of the multiple channel wavelength slots, rather thanrequiring different, channel slot-specific systems to be designed,manufactured, and inventoried for each of the hundreds of wavelengthslots in a given WDM system. From the standpoint of the operator, itwould be desirable to have the ability to receive some wavelengthassignment, then have a generator produce the optical signal carriersignal into that channel assignment on-the-fly. Finally, in higherfunctionality systems such as wavelength add/drop devices, wavelengthtunability is critical to facilitate dynamic wavelength routing, forexample.

SUMMARY OF THE INVENTION

As tunable laser systems become more integrated and physically compact,a problem arises in that the spacing between the longitudinal lasercavity modes can be on the order of the proposed spectral spacingsbetween channels. As a result, even if a given laser system can be tunedto have gain at the allocated channel wavelength, a longitudinal cavitymode for the laser cavity may not exist at that desired wavelength. Inother words, two things must co-exist for a laser to generate light atthe desired wavelength: 1) the cavity must have gain at that wavelength;and 2) the cavity must also have a longitudinal mode corresponding tothe wavelength. Conventional lasers for WDM systems typically do nothave provisions for tuning the cavity length to shift a cavity mode tothe desired wavelength, actively.

In general, according to one aspect, the invention features asemiconductor tunable laser system. The system has a semiconductor chip,which functions as a laser gain medium within the laser cavity. Atunable Fabry-Perot filter is also provided within the laser cavity forselecting a wavelength of operation of the laser system. Finally, acavity length modulator is included to modulate an optical length of thelaser cavity at least over a distance corresponding to the spacingsbetween the longitudinal modes of the laser cavity. Thus, according tothe present invention, the tunable Fabry-Perot filter allows the lasercavity to have gain at the desired wavelength of operation, while thecavity length modulator tunes the laser cavity length such that alongitudinal cavity mode exists at the desired wavelength of operation.

According to the preferred embodiment, a controller is further providedthat collectively tunes the Fabry-Perot laser to the desired wavelengthof operation, and then controls the cavity length modulator so that alongitudinal mode of the cavity is controlled to reside at the desiredwavelength of operation.

Further, in the preferred embodiment, the semiconductor chip comprises asemiconductor optical amplifier chip that functions as the laser gainmedium. Further, according to the preferred embodiment, the tunableFabry-Perot cavity comprises serial first and second partial reflectorsand some means for changing an optical distance between the tworeflectors. In the present implementation, an electrostatic deflectionsystem is used whereby one of the reflectors is moveable and thecontroller modulates an electrical field between electrodes on thestationary and moveable reflectors to thereby change the distancebetween the stationary reflector and the movable reflector.

An electrostatic system is also preferably used to modulate the lasercavity length. In the present implementation, a movable, reflectivemembrane defines one end of the cavity. An electrostatic field controlsthe distance between the moveable membrane and a stationary electrode.

Preferably, the tunable laser system is integrated into a singlehermetically sealed package. Specifically, the semiconductor chip,tunable Fabry-Perot filter, and the cavity length modulator are thenattached or bonded to an optical bench, which is sealed within thepackage. As is common, a fiber pigtail enters the package via a fiberfeed-through to connect and terminate above the bench to receive thelaser beam from the laser cavity.

In the preferred embodiment, a wavelength locker system is further used.It has a differential wavelength filter and a multi-element detector.The controller then modulates the Fabry-Perot cavity to control thewavelength in response to the signal received from the multi-elementdetector.

In general, according to another aspect, the invention can also becharacterized as a process for tuning a semiconductor laser system. Thisprocess comprises amplifying optical energy in a laser cavity with asemiconductor laser gain medium and then selecting a wavelength ofoperation of the laser cavity by tuning a Fabry-Perot filter within thelaser cavity. Further, an optical length of the laser cavity ismodulated over about the spacing between the longitudinal cavity modes.As described previously, the length can be tuned so that a cavity moderesides at the desired wavelength of operation.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a perspective view of a tunable laser system of the presentinvention;

FIG. 2 is a perspective, close-up view showing the wavelength lockersystem of the present invention;

FIG. 3 is a perspective view showing the details of the multi-elementdetector of the wavelength locker system of the present invention;

FIG. 4 is a block diagram/schematic view showing the stepped etalon,multi-element detector, and the electronics for controlling thesemiconductor laser system of the present invention;

FIG. 5 is a spectral plot of power as a function of wavelength(arbitrary units) showing the multiple spectral filteringcharacteristics of the differential wavelength filter system and therelationship to a wavelength of operation of the laser system;

FIG. 6 is a block/schematic view of another embodiment of the wavelengthlocker system utilizing a coarse and fine etalon system of the presentinvention;

FIG. 7 is a close-up view showing the semiconductor optical amplifierportion of the tunable laser of the present invention and the lasercavity of the present invention;

FIG. 8 is a perspective view showing the tunable Fabry-Perot cavity andthe cavity length modulator of the present invention;

FIG. 9 is a perspective view showing the integration of the tunableFabry-Perot cavity and cavity length modulator according to the presentinvention;

FIG. 10 is a schematic block diagram illustrating the electronic controlscheme according to the present invention for the tunable laser;

FIG. 11 is a spectral plot of power as a function of wavelength(arbitrary units) showing the relationship between the wavelength ofoperation or passband of the Fabry-Perot filter and the cavity modes ofthe laser cavity of the tunable laser prior to compensation by thecavity length modulator; and

FIG. 12 is a plot of free spectral range in GHz as a function of cavityoptical length in millimeters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a tunable laser system 100, which has been constructedaccording to the principles of the present invention.

Generally, the tunable laser system 100 comprises a hermetic package110. Typically, this hermetic package is a standard pin package, such asa butterfly package or dual, in-line (DIP) package. Presently, astandard sized packaging platform is used, which has a hermetic cavityof less than 2.0 centimeters (about 0.75 inches) in length and less than1.5 centimeters (cm) (approximately 0.5 inches) in width.

An optical bench or submount 105 is installed within the hermeticpackage 110. The optical components of the tunable laser system 100 areinstalled on this bench. In the preferred embodiment, the bench isconstructed from a mechanically and temperature stable substance, suchas aluminum nitride, silicon, silicon oxide, or beryllium oxide invarious implementations. The bench is typically installed in the packageon a thermo-electric cooler.

An optical fiber pigtail, such as a length of single mode optical fiber,112, enters the hermetic package 110 through a fiber feed-through 114.Typically, the pigtail 112 passes through a ferrule in the feed-through.In one implementation, single mode polarization-maintaining fiber isused.

The termination or endface 122 of the fiber pigtail 112 is installed orattached to the bench 105 using a fiber mounting and alignment structure210. This mounting structure 210 connects the fiber pigtail 112 suchthat the endface 122 is secured in a rigid connection to the bench 105and supported above the bench. The optical signal is coupled into thisfiber pigtail 112 via the fiber endface 122 and transmitted outside ofthe tunable laser system 110. Preferably, deformable mounting structuresare used to enable active or passive alignment during system manufactureor calibration after an in-service period.

The optical components of the tunable laser system 100 are divided intothree subsystems. The tunable semiconductor laser 116 generates theoptical signal in the form of an output beam. A wavelength lockersubsystem 118 determines a wavelength of the optical signal using aportion light, preferably from the output beam, from the tunable laser116. The output coupling section 120 couples the remainder, or mainportion, of the optical signal beam into the fiber pigtail 112 fortransmission out of the system 100. An isolator 117 of the tunable laser116 is preferably installed between the laser and the output couplingsection 120/locker 118. This isolator prevents backreflections into thelaser 116 to help stabilize its operation.

FIG. 2 shows the details of the wavelength locker system 118 of thetunable laser system 100. Specifically, beam splitter 310 splits theoutput beam 410 from the tunable laser system 116 such that a portion312 of the beam is provided to the locker system 118 as its input beam.

The input beam 312 is filtered by what is termed a differentialwavelength filter system. This system applies multiple spectralfiltering characteristics to input beam 312.

In the preferred embodiment, the differential wavelength filter system314 is a stepped etalon system that has bulk material 318 providing thegeneral free spectral range of the etalon. Further, one end of theetalon has stepped sections that have different heights with respect toeach other. These steps have the effect of fine-tuning the net freespectral range of the filter and yield the multiple spectralcharacteristics of the system.

The filtered beam 320 from the differential wavelength filter system isthen received by a multi-element detector 316. This detector is alignedwith the differential wavelength filter to detect the magnitude of thebeam, and specifically, different portions of the beam to which thedifferent spectral filtering characteristics have been applied.

FIG. 3 shows the multi-element detector 316. Specifically, in thepresent implementation, the multi-element detector 316 is aquad-detector with the individual detector elements 322, 324, 326, and328 being commonly mounted on a single substrate-carrier that is ismounted in an orthogonal relationship to the plane of bench 105.Specifically, detector 316 comprises four discrete, photosensitiveelements 322, 324, 328, and 326 that are laid out in square, gridfashion on the carrier 329. Each element 322-328 of the quad detectorhas a corresponding bond pad electrode 330, 332, 334, 336 withconnecting wire traces formed on the substrate (see reference 327, forexample). In this way, the magnitude of the optical energy impingingupon each element of the quad detector is separately, electricallydetected via wire-bonding to the four bond pads 330-336.

FIG. 4 illustrates the operation of the stepped etalon 314 andwavelength locker system opto-electronics. Specifically, the beam 312from the tunable laser system 116 enters the stepped etalon 314. Oneface 340 of the etalon 314 has a standard, partially reflecting surface.The beam propagates through the bulk portion 318 of the etalon to theother partially reflecting surface of the etalon, which is stepped.Specifically, there are, in the illustrated embodiment, four separatefaces 342, 344, 346, 348 at the other end of the etalon, each at adifferent optical distance from the first face 340.

Each one of surfaces 342, 344, 346, 348 corresponds to a differenteffective length etalon. Specifically, the etalon length differencebetween surface 342 and 344, in one embodiment is approximately ⅛ of thewavelength of the center operational wavelength of the system (⅛λ). Thedistance between the second surface 344 and third surface 346 is ⅛λ, andthe distance between the third surface 346 and the fourth surface is ⅛λ.As a result, the stepped etalon provides multiple spectral filteringcharacteristics.

The magnitude of the light transmitted through the separate filtercharacteristics is then detected by the elements 322-328 of the quaddetector. Specifically, the signals from the quad detector elements areamplified by an amplifier bank 350 and provided to either separateanalog-to-digital (A/D) converters or one shared A/D converter circuit352. The magnitude of the signals from the elements is then received bya wavelength controller 354.

The wavelength controller 354 compares the absolute and relativemagnitudes of the is signals from the detectors 322-328. The wavelengthcontroller 354 determines the wavelength of operation by reference to aquad-detector to wavelength mapper 356.

In one embodiment, this mapper 356 is an equation or translationfunction that relates the detector element response to a wavelength ofoperation of the tunable wavelength circuit 116. This equation ortranslation function was determined for the system at the time ofmanufacturing or later during a calibration or recalibration process. Inanother embodiment, the mapper 356 is implemented as a look-up table(LUT), which similarly records the quad detector element response towavelength relationship. Based on this information, the wavelengthcontroller then tunes the wavelength of operation of the tunable laser116. In a simple embodiment, this can be accomplished by adjustinginjection current or current to the thermoelectric cooler to therebycontrol the tunable laser's wavelength of operation. As described later,in the present embodiment, it is also used to tune a cavity element ofthe tunable laser 116.

In other embodiments, the sum signal from the elements of thequad-detector is used to determine amplitude, allowing the controller tofurther provide amplitude feedback control.

This information is also used to tune the cavity length.

FIG. 5 is a spectral plot illustrating the operation of the wavelengthlocker 118. Specifically, the differential wavelength filter 314, in itsstepped etalon configuration, applies essentially four separatefiltering characteristics or transfer functions 360, 362, 364, 366corresponding to the endfaces 342, 344, 346, and 348 of the steppedetalon 314. These multiple spectral filtering characteristics repeatthemselves spectrally in increments of the free spectral range 368determined by the bulk 318 of the etalon 314. As a result, each of thedetector elements detects a different filtered version of the inputbeam.

Specifically, in the illustrated case, if the wavelength of the inputsignal 312 is as illustrated, detector 322 receives a relatively strongsignal since the emission from the tunable laser 312 is near the peak ofits corresponding filtering characteristic 364. Detector element 328receives a smaller signal since the emission wavelength is a greaterspectral distance from the peak of its corresponding filteringcharacteristic 366. The other detectors receive relatively smallsignals.

The wavelength controller 354 then compares the relative intensities ofthe signals from the various detector elements and then uses thewavelength mapper 356 to determine the wavelength of operation of thesemiconductor laser system 116. Generally, for the wavelength controllerto resolve the wavelength of the signal, it must receive a sufficientsignal from at least two detectors since a given voltage from a singledetector can be mapped to two distinct wavelengths as illustrated by thefact that line 322 intersects characteristic 364 at two differentwavelengths.

FIG. 6 illustrates another embodiment of the wavelength locker system118. In this configuration, the input signal 312 from the tunablesemiconductor laser 116 is again divided by a second beam splitter 610of the locker and then redirected by an optional fold mirror 612 so thattwo discrete beams enter the differential wavelength filter system 314.This second embodiment differential filter system, however, comprisestwo stepped etalons 618, 614, and two separate multi-element quaddetectors 616 and 316.

In this embodiment, etalon 614 corresponds to a coarse filtering etalon.Specifically, it is again an etalon with a multiple steps at its outputfacet. The bulk material, however, has a free spectral rangecorresponding to a relatively wide or coarse spectral band. In thepresent embodiment, it has a free spectral range of approximately 50 to150 nm. Specifically, it is anticipated that its free spectral rangeexceeds 100 nm. This allows it to have a free spectral range that coversthe entire wavelength band associated with common WDM systems. Suchsystem bands stretch from about 1500 to 1600 nm. In this way, thecorresponding coarse multi-element detector 616 can be used to determinea general or coarse wavelength of operation of the tunable laser 116.

The fine detecting etalon 618 has a much smaller spectral range.Typically, its spectral range is less than 10 nm and most commonly lessthan one nm. This fine filtering etalon in combination with the finemulti-element detector 316 allows for fine wavelength detection of thebeam 312 from the tunable laser 116. Specifically, because of its muchnarrower free spectral range, it can more precisely determine thewavelength of operation.

As a result, the coarse and fine multi-element detectors are used incombination to first determine the general wavelength of operation withthe coarse multi-element detector, and thereby determine which mode ofthe fine-element detector etalon is being excited. Then, the finemulti-element detector is then used to control precisely the wavelengthto better than 0.1 nm accuracy.

In other embodiments, one stepped coarse etalon and one standard,non-stepped, fine etalon are used in a cascade arrangement. One quaddetector detects the output of the first etalon, and the other detectsthe signal after propagating through both etalons

FIG. 7 shows the specifics of the preferred embodiment of the tunablelaser system 116. Generally, the tunable laser 116 comprises asemiconductor gain medium 422. In the preferred embodiment, the gainmedium is, for example, a semiconductor optical amplifier (SOA) chipconstructed from InGaAsP/InP, for example. As is common, the chip 422has an anti-reflection coated rear facet. The front facet is coated todefine the front reflector of the laser cavity. In one embodiment, thefront facet coating provides between 5 and 15% power reflectivity. Thetunable laser further comprises a tunable Fabry-Perot filter or cavity410 and a laser cavity length modulator 412 with polarization rotatorsmaterial, such as quarter-wave plates or Faraday rotators, to preventamplification of rejected light from the filter in the chip 422.

According to the preferred implementation, the SOA chip 422 ispolarization anisotropic. As such, it only amplifies light that iseither TM or TE polarized. As is commonly known, this is accomplished bycontrolling the strain in the quantum well layers of the chip duringwafer-stage epitaxial growth to either have tensile strain (TM) orcompressive strain (TE).

Utilizing a polarization anisotropic SOA chip runs contrary to thetypical design of SOA's. When used as amplifiers, it is typicallydesirable to amplify light regardless of its polarization. In thecurrent invention, the polarization anisotropy, however, is used toavoid the amplification of light rejected from the tunable filter 410 asdescribed below.

Light is coupled out and into the SOA chip 422 by front and rear facetfocusing lenses 418, 416, which are supported on respective mountingstructures 420 and 414. These mounting structures allow for the x-, y-and z-axis alignment of the lenses 418 and 416 relative to the facets ofthe chip 422 after construction of the system to improve the couplingefficiency both into and out of the SOA chip 422.

FIG. 8 illustrates the details of the cavity length modulator. Themodulator is constructed on a rotator-material substrate 436, which isattached to bench 105. On the back surface of the substrate, a highlyreflecting membrane 430 is fabricated. In the preferred embodiment, thismembrane is a flexing membrane constructed from a metal such as aluminumor semiconductor material such as silicon.

This membrane is translated axially, or in the direction of the z-axisby applying an electrical potential between the membrane 430, via anelectrical connection to pad 436, and a stationary membrane electrode432, via pad 434. As a result, by controlling the electrostatic voltagelevel between the membrane 430 and the stationary membrane electrode432, the membrane is translated in a controlled fashion. Since themembrane is highly reflecting or HR coated and defines the cavitylength, the length of the laser cavity is thus modulated.

FIG. 9 illustrates the tunable Fabry-Perot filter 410 and itsrelationship to the cavity length modulator 412. Specifically, thetunable Fabry-Perot (FP) filter or cavity 410 is constructed between tworotator material substrates 440, 436. The FP cavity comprises a frontpartially reflecting surface, which is constructed from a movablepartially reflecting membrane 442. This membrane is supported on supportposts 450 in proximity to a stationary electrode 444, which isfabricated or deposited on the proximal side of the second rotatormaterial substrate 436. The stationary electrode also functions as thesecond partially reflecting surface of the FP cavity. The membrane ispreferably constructed from silicon, for example. The posts alsofunction as stand-offs with respect to the two rotator substrates 440,436 to thereby define the length of the FP cavity demarcated by themembrane 442 and the electrode 444.

By applying a voltage between bond pad 448 and bond pad 446, anelectrostatic field is generated between partial reflecting membrane 442and the electrode 444. This field flexes the membrane to provide z-axismovement of the reflector electrode 442 to change the distance betweenthe membrane 442 and the partial reflector-electrode 444 to therebychange the filtering characteristic or passband and thus tune theFabry-Perot filter 410. This controls the spectral location of the lasercavity's gain.

In the illustrated embodiment, the bond pads 448, 446 also function asx-axis alignment stubs for facilitating construction of the Fabry-Perotcavity 410/cavity length modulator 412, typically prior to installationon the bench 105. Specifically, they facilitate the attachment of therotator material substrates to each other by allowing the heightalignment between substrates 436, 440.

The rotator material substrates 440 and 436 co-act to prevent rejectedlight from the FP cavity from being amplified in the SOA chip 422,whereas passband light is amplified.

Specifically, in the preferred embodiment, the material and thickness ofthe substrates 440, 435 is selected such that they function as Faradayrotators that rotate the polarization of transmitted light by 45°.Specifically, light emitted from the rear facet of the SOA chip 422 ispolarized either orthogonally or parallel, depending on the straincharacteristics of the chip's quantum well(s). The light is subsequentlyrotated 45° before entering the FP cavity 410 by the rotator materialsubstrate 440.

The FP cavity divides the light into the resonant and off-resonantcomponents. That is, light outside the passband of the FP cavity isreflected to return to the SOA chip via a second pass through the firstrotator substrate 440. This rotates the light a further 45°. Since thelight rejected by the FP cavity is now orthogonal to the light emittedby the chip 422 it passes through the chip without amplification becauseof the designed polarization anisotropy. The low reflectivity of thechip front facet prevents reflection of most of the light back into thelaser cavity and consequently the lasing in the double round-tripconfiguration.

In contrast, light at the FP cavity's passband, i.e., the resonantlight, passes through the FP cavity and enters the second rotatormaterial substrate 436 and is reflected by the membrane 430 to return.As a result, it is rotated 45° twice in the first substrate 436 and 45°again in the second substrate 440 after passing back through the FPcavity. Thus, this returning passband light, in contrast to the rejectedlight, is polarized parallel to the light generated in the SOA chip andthis is amplified, resulting in the tunable laser functionality.

In one embodiment the rotator material substrates are constructed fromlatched garnet crystal material as sold by Mitsubishi Gas Chemical Co.,Inc.

In another embodiment, quarter-wave plates are used as the substrates440 and 436. Quarterwave plates can be used since the polarization ofthe light from the SOA chip is known and invariant. Specifically, inthis embodiment, light passes through the first quarterwave platepolarization rotator 440 and is converted to circularly polarized light.Rejected light from the filter passes back through the firstpolarization rotator to be converted to light that is orthogonal inpolarization to the light from the SOA chip, and thus it is notamplified. In contrast, passband light passes through tunable filter tothe second quarterwave plate polarization rotator 436 and converted toorthogonally polarized light and reflected. As it returns, it isconverted to a parallel polarization after successively passing throughthe second and first quarterwave plates.

In still another embodiment, subwavelength period grating substrates areused as polarization rotators. Such gratings, as described in AppliedPhysics Letters 42 (6), Mar. 15, 1983, page 492, do not diffract thelight, but instead operate as a homogenous birefringent material torotate the polarization of the beam. In one implementation, the gratingsare etched, or otherwise formed, onto a side of the bulk substrates 440,436 to a depth required for quarterwave operation. In still otherembodiments the polarization rotation is performed by liquid crystal,preferably in photopolymerizable polymer utilizing photoalignment.

FIG. 10 illustrates the electronic control scheme for the tunable laser100. Specifically, a locker system 118 is used to generate a signalindicative of the wavelength of operation of the tunable laser section116. In one implementation, this locker is configured as describedpreviously herein. In any case, however, the information from thewavelength locker system 118 is then fed to the wavelength controlsystem 354.

More specifically, in one embodiment, the wavelength controller 354comprises a microprocessor 718. The microprocessor 718 controls thetunable filter 410 and the cavity length modulator 412 via a filterdriver 714 and length modulator driver 716. In one implementation, eachof these drivers 714, 716 comprises a digital-to-analog converter thatconverts a digital voltage setting from the microprocessor 718 into anelectrostatic drive voltage for the tunable filter 410 and the cavitylength modulator 412.

In one embodiment, the wavelength controller further comprises acapacitance proximity detector 710 and capacitance-to-wavelength look-uptable or mapper 712. Specifically, the proximity detector detects acapacitance between the membrane 442 of the Fabry-Perot filter and thestationary electrode 444. This detected capacitance is then converted toa current wavelength of operation of the tunable filter by using thedetected capacitance as an address into the look-up table or mapper 712.The mapper contains the relationship between the Fabry-Perot filter'spassband and the membrane-electrode capacitance. By detecting thecapacitance, the current passband of the tunable filter is found withoutan optical reference signal. This information is passed to thecontroller.

Of course, the mapper can alternatively be implemented as an equation orfunction instead of a look-up table. This is a more compactimplementation especially where the relationship is linear or nearlinear.

FIG. 11 illustrates the operation of the semiconductor tunable lasersystem and the control of the tunable Fabry-Perot cavity 410 and thecavity length modulator 412 by the microprocessor 718. Specifically, thedesired wavelength of operation λ₀ is achieved by tuning the Fabry-Perotcavity such that the filtering characteristic 1110 of the Fabry-Perotcavity has a peak at the desired wavelength λ₀. This is achieved in oneembodiment by reference the capacitance to wavelength look-up table ormapper 712. Optical reference signals are used in other implementations.

As is known, the laser cavity has discrete longitudinal modes ofoperation 1112. The modal separation is related to the cavity length ofthe laser:

Δλ=λ₀ ²/(2 nL)

where L is the length of the cavity and n is the refractive index.

According to the present invention, the Fabry-Perot cavity 410 is firsttuned to the desired wavelength of operation, then the cavity lengthmodulator 412 is driven to vary the physical length of the laser cavityby an amount that is typically less than one wavelength of light at theoperational wavelength. The operational wavelength is typically lessthan 1610 nm. Minimally, to render a mode coincident with the desiredwavelength of operation, the length should only need to be adjusted byone half of a wavelength either larger or smaller.

Typically, this one-half wavelength distance is less than 1000 nm.

In FIG. 11, no mode is aligned with the peak of the Fabry-Perot cavityfiltering characteristic 110. This can result in either mode-hoppingbetween modes for which the laser system has gain, or suboptimal poweroutput because of the difference between the peak of the transferfunction of the passband of the Fabry-Perot cavity and the next closestcavity mode.

This fine intercavity mode tuning is achieved in one embodiment byreference to the absolute wavelength as detected by the locker 118. Inanother embodiment, the cavity length is adjusted until the output poweris maximized, which will occur when a cavity mode is centered at thecenter wavelength of the FP filter 410 and/or there is no mode-hoppingbetween two or more competing modes within the passband. In either case,one of the cavity modes 112 is placed to reside at the peak of thepassband of the Fabry-Perot cavity 410. In this way, the presentinvention makes the necessary correspondence between the pass wavelengthof the tunable Fabry-Perot cavity and a longitudinal mode placement ofthe laser cavity.

The present invention is most applicable to tunable lasers having shortlaser cavities. According to the present invention, the length of thelaser cavity (See distance “L” in FIGS. 1 and 10) is preferably lessthan three centimeters. In the preferred embodiment, it is less than onecentimeter. This length yields a modal separation of greater than a 3GigaHertz (GHz). Preferably, the spacing is greater than 10 GHz, with aspacing of better then 20 to 30 GHz being ideal.

FIG. 12 illustrates the relevance of the short laser cavity L to modalspacing. Laser cavities of three centimeters and longer have freespectral ranges of approximately 3 GHz and less. With such close modalspacing, the need for cavity length adjustment can be avoided.Specifically, modern dense WDM (DWDM) systems contemplate channelspacings of 100 GHz with reference to the Lα, Cα, and Sα bands in theITU 100 GHz grid. Denser systems contemplate 50 GHz channel separationsas specified in the Lβ, Cβ, and Sβ band of the 50 GHz offset ITU grid.At these channel separations, transmitter frequency must be specified totypically about +/−5 GHz. The modal spacing in a four centimeter orlonger cavity can inherently accommodate these close channel spacing,but short cavities of one centimeter or less as preferred in the presentinvention yield mode spacings of greater than 10 GHz. Thus, the shortcavity lasers, such as the instant invention, necessitate cavity lengthcompensation as achieved with the cavity length modulator 412 of thepresent invention.

The preceding analysis suggests that longer cavity lasers are in factdesirable, since they avoid the need for cavity length adjustment inDWDM systems. Two factors, however, advocate for shorter cavityconfigurations: 1) device size; 2) an FP filter finesse requirements.

DWDM systems comprising up to one hundred channels or more require acorresponding number of tunable transmitters. In such complex systems,individual device sizes become critical and there is a need to havesmall transmitter form factors. In short, smaller is typically betterfrom the standpoint of integration.

Higher finesse Fabry-Perot filters are required as cavity mode spacingdecreases. The bandwidth of the Fabry-Perot filter must be narrow enoughto select and restrict the tunable laser to single mode operation. Whenmodes are tightly spaced, high finesse Fabry-Perot filters are requiredto avoid mode hopping or multi-longitudinal mode operation.

In contrast, when a shorter cavity is used, a much lower finesse or“quality” Fabry-Perot filter can be used that has a wider bandwidth endto 100 GHz. The ability to use a lower finesse Fabry-Perot filterdecreases alignment tolerances between the SOA 422 and the filter,increases the gain of the laser cavity, and the avoids the requirementto fabricate very thick, multi-layer dielectric coatings on the surfacesof the Fabry-Perot filters. These factors substantially reduce cost andmanufacturing complexity of the device.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A semiconductor tunable laser system, comprising:a semiconductor chip functioning as a laser gain medium within a lasercavity; a tunable filter within the laser cavity for selecting awavelength of operation of the laser system; a cavity length modulatorthat modulates an optical length of the laser cavity; and a controllerthat tunes the tunable filter to a desired wavelength of operation andcontrols the cavity length modulator so that a longitudinal mode of thelaser cavity resides at the desired wavelength of operation.
 2. Asemiconductor tunable laser system as claimed in claim 1, wherein thesemiconductor chip comprises a semiconductor optical amplifier chip witha reflective front facet and antireflection coated rear facet.
 3. Asemiconductor tunable laser system as claimed in claim 1, wherein thetunable filter is a Fabry-Perot filter and comprises serial first andsecond partial reflectors and means for changing an optical distancebetween the reflectors.
 4. A semiconductor tunable laser system asclaimed in claim 1, wherein the tunable filter is a Fabry-Perot filterand comprises a stationary partial reflector, a movable partialreflector including a reflector electrode, and a stationary electrode,wherein the controller modulates an electric field between the reflectorelectrode and the stationary electrode to change a distance between thestationary partial reflector and the movable partial reflector andthereby select a wavelength of operation of the laser system.
 5. Asemiconductor tunable laser system as claimed in claim 1, wherein thecavity length modulator comprises a movable reflector defining one endof the laser cavity.
 6. A semiconductor tunable laser system as claimedin claim 5, wherein the movable reflector is highly reflective to definea back-end of the cavity.
 7. A semiconductor tunable laser system asclaimed in claim 5, wherein the movable reflector comprises a movablemembrane including a membrane electrode and a stationary electrode,wherein an electric field between the membrane electrode and thestationary electrode displaces the movable membrane to thereby change alength of the cavity.
 8. A semiconductor tunable laser system as claimedin claim 1, further comprising: an hermetic package; an optical benchsealed within the package, the semiconductor chip, the tunable filter,and the cavity length modulator being attached to the optical bench. 9.A semiconductor tunable laser system as claimed in claim 8, furthercomprising a fiber pigtail entering the package via a fiber feed-throughto connect to the bench and terminate above the bench to receive a laserbeam from the laser cavity.
 10. A semiconductor tunable laser system asclaimed in claim 9, further comprising an output-focusing lens forcoupling the laser beam into the fiber pigtail.
 11. A semiconductortunable laser system as claimed in claim 8, further comprising focusinglenses installed on the bench at either end of the semiconductor chip tocouple optical energy into and out of the chip.
 12. A semiconductortunable laser system as claimed in claim 1, wherein the cavity lengthmodulator changes a length of the laser cavity over of a distancecorresponding to about the wavelength of operation.
 13. A semiconductortunable laser system as claimed in claim 1, wherein the cavity lengthmodulator changes a length of the laser cavity over of a distancecorresponding to about one half of the wavelength of operation.
 14. Asemiconductor tunable laser system as claimed in claim 1, furthercomprising a polarization rotator for rotating a polarization of lightrejected by the tunable Fabry-Perot filter to prevent amplification ofthe rejected light.
 15. A semiconductor tunable laser system as claimedin claim 1, further comprising an additional rotator for rotating apolarization of light passed by the tunable Fabry-Perot filter to enableamplification of the passband light.
 16. A semiconductor tunable lasersystem as claimed in claim 1, wherein the tunable filter has a lowfinesse being able to limit the operation of the laser cavity to asingle mode of cavity mode spacings of greater than 3 GigaHertz.
 17. Asemiconductor tunable laser system as claimed in claim 1, wherein thetunable filter has a low finesse being able to limit the operation ofthe laser cavity to a single mode of cavity mode spacings of greaterthan 10 GigaHertz.
 18. A semiconductor tunable laser system as claimedin claim 1, wherein the tunable filter has a low finesse being able tolimit the operation of the laser cavity to a signal mode of cavity modespacings of greater than 20 GigaHertz.
 19. A semiconductor tunable lasersystem as claimed in claim 1, wherein a cavity mode spacing of the lasercavity is greater than 3 GigaHertz.
 20. A semiconductor tunable lasersystem as claimed in claim 1, wherein a cavity mode spacing of the lasercavity is greater than 10 GigaHertz.
 21. A semiconductor tunable lasersystem, comprising: an optical bench; a semiconductor chip, installed onthe optical bench, functioning as a laser gain medium within a lasercavity; a tunable Fabry-Perot filter, installed on the optical bench,within the laser cavity for selecting a wavelength of operation of thelaser system; and a wavelength locker system, installed on the opticalbench, including: a differential wavelength filter system that appliesmultiple spectral filtering characteristics to a beam from the lasercavity; a multi-element detector that is aligned to the differentialwavelength filter to detect a magnitude of the beam after being filteredby the multiple spectral filtering characteristics of the differentialwavelength filter; and a controller that modulates the tunableFabry-Perot cavity to control a wavelength of the system in response todifferences in magnitude of the filtered beam detected by themulti-element detector.
 22. A semiconductor tunable laser system asclaimed in claim 21, further comprising a cavity length modulator thatmodulates a length of the laser cavity over of a distance greater than aspacing between longitudinal cavity modes of the laser cavity.
 23. Asemiconductor tunable laser system as claimed in claim 21, wherein thedifferential wavelength filter system comprises a stepped etalon.
 24. Asemiconductor tunable laser system as claimed in claim 23, wherein thestepped etalon comprises one step.
 25. A semiconductor tunable lasersystem as claimed in claim 23, wherein the stepped etalon comprises twosteps.
 26. A semiconductor tunable laser system as claimed in claim 21,further comprising a beam splitter outside of the laser cavity of thesemiconductor laser that provides a portion of the output of the laseras the beam received by the differential wavelength filter system.
 27. Aprocess for tuning a semiconductor laser system, comprising: amplifyingoptical energy in a laser cavity with a semiconductor gain medium;selecting a wavelength of operation of the laser cavity by tuning aFabry-Perot cavity within the laser cavity; and modulating an opticallength of the laser cavity over of about one half of the wavelength ofoperation so that a longitudinal mode of the laser cavity resides at thewavelength of operation selected by the Fabry-Perot cavity.
 28. Aprocess as claimed in claim 27, further comprising: tuning theFabry-Perot cavity to the wavelength of operation; and then modulatingthe optical length of the laser cavity so that the longitudinal mode ofthe laser cavity resides at the wavelength of operation.
 29. A processas claimed in claim 27, wherein tuning the Fabry-Perot cavity comprisesmodulating an electric field to change a distance between a stationarypartial reflector and a movable partial reflector.
 30. A process asclaimed in claim 27, wherein step of modulating the optical length ofthe laser cavity comprises moving a movable reflector defining one endof the laser cavity.
 31. A process as claimed in claim 30, wherein stepof moving the movable reflector comprises modulating an electric fieldbetween movable reflector and a stationary electrode to displace themovable reflector.
 32. A process as claimed in claim 27, furthercomprising selecting a length of the laser cavity so that cavity modespacing is greater than 3 GigaHertz.
 33. A process as claimed in claim27, further comprising selecting a length of the laser cavity so thatcavity mode spacing is greater than 10 GigaHertz.
 34. A semiconductortunable laser system, comprising: a semiconductor chip functioning as alaser gain medium within a laser cavity; a tunable filter within thelaser cavity for selecting a wavelength of operation of the lasersystem; and a cavity length modulator that modulates an optical lengthof the laser cavity; a filter driver for generating a control voltageapplied to the tunable filter; a modulator driver for generating avoltage applied to the cavity length modulator; and a microprocessor forcontrolling the filter driver and the modulator driver.
 35. Asemiconductor tunable laser system as claimed in claim 34, furthercomprising a proximity detector for determining a cavity length of thetunable filter.
 36. A semiconductor tunable laser system as claimed inclaim 35, wherein the proximity detector detects a capacitance between amembrane and stationary electrode of the tunable filter.
 37. Asemiconductor tunable laser system as claimed in claim 34, furthercomprising wavelength to proximity mapper that correlates the detectedcavity length to passband of the tunable filter.